There are only three ways to do this.
1. Take it to your local Fiat dealership with the registration document and they will give you the code for free based on the vehicle VIN Number.
2. Look for a small white sticker in the glove compartment or the centre console. When the car is new, this sticker is supplied with the manual with those instructions.
3. You pay an online Radio Code Decoding company. Many of them say 'Free', but most charge about $15 or £12 UKP. The major sites are trustworthy and will work with you, or refund if the code doesn't work. The sites need the Serial Number of the radio which is either engraved to the radio chassis, or on a sticker at the rear or sides hidden from view.